Electrodeless pulsed illuminator

ABSTRACT

A gaseous pulsed illuminator utilizing a halogen gas. No electrodes are required to strike the discharge and accordingly highly reactive gases having short luminous decay times may be employed. The excitation power is a radiofrequency source coupled through a waveguide or other RF transmission line.

United States Paten Goldie et al. Y 51 Mar. 7, 1972 ELECTRODELESS PULSED 3,280,364 10/1966 Sugawara et al ..315/39 X [LLUMINATOR 3,313,979 4/1967 Landauer ..315/39 3,374,393 3/1968 Bramley ..3 15/] H X [72] Inventors: Harry Gold e, Randallstown; Michael A. 3 431 4 1 9 9 Dodo et 1 "315/39 Goldman, Plkesvllle, both of Gerald 3,479,555 11/1969 Garwin ..315/39 I. Klein, Westbury, NY. [73] Assignee: Westinghouse Electric Corporation, Pitt- FOREIGN PATENTS OR APPLICATIONS burgh, P 215,373 6/1968 Australia ..3l5/l H [221 filed Mar-24,1969 Primary Examiner-Eli Lieberman [21] Appl' 0 71 Assistant ExaminerSaxfield Chatmon, Jr.

Att0rneyF. H. Henson, E. P. Klipfel and J. L. Wiegreffe [52] US. Cl. ..3l5/39, 315/111, 333/99 PL [57] BSTRACT [51] lnt.Cl. ..H01j7/46,l-l0ij 19/80 H [58] Field of Search "M31 5/39 1 1 1; 333,99 PL A gaseous pulserl illuminat or utilizing a halogen gas. No electrodes are required to strike the discharge and accordingly [56] References Cited highly reactive gases having short luminous decay times may be employed. The excitation power is a radiofrequency source UNITED STATES PATENTS coupled through a waveguide or other RF transmission line.

6/1953 Goldstein et al ..3l5/39 X 3 Claims, 4 Drawing Figures Patented March 7, 1972 TIME (I SEC/CMI P= mowms 'MAx ELECTRODELESS PULSED ILLUMINATOR This invention relates to a gaseous discharge device of the type adapted to produce pulsed, incoherent illumination. The invention further relates to such a source of illumination wherein the luminous decay is extremely rapid to thereby admit of a close time correspondence between the energization/deenergization of the power source and the commencement/termination of illumination from the gas.

The general mechanism of emission of luminous energy from a gas which has been excited has been the subject of research over a great many years and is generally well understood. According to such accepted views, mechanical,

thermal, or electrical energy is applied to a gas and the energy levels of certain of the electrons in the gas increase from their initial or ground levels to higher energy levels. Almost immediately thereafter a dropping back from higher to lower levels of these electrons occurs, this dropping back accompanied by the emission of light. The loss of energy in returning to the ground levels is represented by the energy of the emitted light.

One source of illumination is that of an arc struck between two electrodes in an envelope filled with a gas. The electrical energy from the spaced electrodes in contact with the gas serves to transmit energy to the molecules in the gas, which in turn emit light by the above-mentioned mechanism. While satisfactory in many respects, an electrical arc illumination suffers from slow electrode erosion under the combination of intense ion bombardment and relatively high temperatures. To inhibit gas cleanup, which occurs as a result of interaction between the energetic particles and the hot electrode surfaces, inert (noble) gases such as xenon, argon and krypton are employed. However, the use of these gases entails acceptance of their slow luminous decay upon the withdrawal or decay of the exciting energy pulse. In certain situations, such as stroboscope studies, it is desired that the illumination from a gaseous source be extinguished as soon as the energizing electrical pulses are stopped.

A halogen gas exhibits the property of rapid cessation of illumination with cessation of the energization. However, halogen gases, i.e., chlorine, iodine, and bromine, are extremely active chemically. The use of such gases in an electrode type of arc discharge has accordingly not been practiced because of the consequent high chemical reaction rate of such gases with the electrodes. In accordance with the practice of this invention, this rapid quench property of a halogen gas, believed to be due to its extremely high electron-attachment coefficient, is realized at high plasma temperatures. Due to the afi'rnity of halogen gas molecules for free electrons (such as the electrons in a gaseous plasma), the rapid rate of free electron disappearance causes the light output to attenuate at approximately the same rate as the exciting pulse. According to the invention, a halogen gas is encapsulated in a container and radio frequency energy applied to energize the gas to luminous, plasma temperatures. In general, the inventionembraces any attaching gas, i.e., one whose outer shell or shells lacks one or two electrons.

In the drawings:

FIG. I is a schematic view illustrating the general arrangement of elements employed for the practice of the invention.

FIG. 2a is a plan view of a quartz capsule containing a halogen gas and mounted in a supporting reflector, and showing an iris slot in a waveguide relative to the capsule.

FIG. 2b is a partial cross section and illustrates a quartz capsule filled with a halogen gas placed in proximity to one end of a waveguide, and illustrates the relation of the waveguide, reflector and slot.

FIG. 3 illustrates a typical observed time relationship between energy input to the gaseous illuminator of this invention and the light increase and decay.

Referring now to FIG. 1 of the drawings, the numeral denotes a radiofrequency source of energy whose output is coupled to a transmission line indicated schematically by numeral 12. The input from the transmission line is fed to a coupling impedance matching network 14 which matches the impedance of the source 10 to that of the load in order that the maximum amount of power be transferred from the source to the load. The numeral 16 denotes a slot formed in one end of a conventional waveguide and communicates with the interior of a reflecting or beam forming surface 18. The numeral 20 schematically designates a discharge illuminating device according to the invention defined by an encapsulated halogen gas. The numeral 22 schematically denotes plasma within the gas arising from the absorption of the radio frequency energy by the gas.

Referring now to FIGS. 2a and 2b, a quartz container 20 contains a quantity of halogen gas 22 which is adapted to form an illuminating plasma 23. The envelope 20 may be formed of quartz or other visible light transparent material and may be held in the illustrated recess or cavity of the reflecting element 18 by means of a suitable adhesive denoted by the numeral 26.

Referring again to FIG. 2b of the drawings, P denotes incoming radio frequency energy, having an electric vector E, transmitted by conventional waveguide means such as 24, or more generally, by and RF transmission line such as schematically designated at FIG. 1 of the drawings. This radiation energy strikes the portion 25 at the end of the waveguide and thence slot 16. The slot is termed a coupling aperture and its function is tointensify the electric field portion of the electromagnetic waves propagating along the waveguide 24 from right to left. The volume within the dashed circle 17 schematically denotes the zone where the mentioned intensification of the electric field occurs. The now intensified oscillating electric field passes through the contiguous wall portion of the capsule 20 and acts on free electrons (beta rays) continuously given off by a small quantity of radioactive material 27 placed within the capsule. The RF electric field-electron interaction causes these electrons to oscillate, increasing their energy until ionization of the halogen gas molecules occurs The direction of oscillation is parallel to the electric vector E, and is shown at the right portion of FIG. 2a. The minimum radiofrequency is hence determined by the minimum spacing of the capsule walls in the direction of electron oscillation. Shortly after initial ionization, the gas absorbs enough energy to form a plasma, here denoted by the numeral 23. While the source 27 may be omitted in theory, thus removing the source of initiatory electrons, practice requires it in order to realize reliable performance and uniform building up of the luminous, plasma glow. By reference to FIG. 2a, it will be observed that the general outline of the plasma follows the shape of the slot 16, although increases in RF power will cause the plasma glow to fill the capsule. When the RF energy is cut off, the electrons which are part of the plasma rapidly revert from their free or disassociated state, due to the affinity of the halogen (attaching) gas for free electrons. The following table, taken from Basic Data of Plasma Physics (2nd Ed.) by S. C. Brown, published by John Wiley Co., illustrates this affinity, especially with respect to the noble gases generally used in glow tube devices.

TABLE I 1 (electron attachment coefficient) Halogcns Noble Gases Chlorine +3.l ev. Argon l ev. Fluorine +2.9 ev. Helium -0.53 ev.

Bromine +3.6 ev. Hydrogen +0.53 ev.

large volumes of plasma, where there are no nearby (container) surfaces for the free electrons to become attached after the removal of the plasma-sustaining energy.

In practice, the capsule 20 may be in the form of a tube, a rectangular parallelpiped, or in the form of a high-Q microwave one-port, glass-lined, mirror-walled cavity. The relationship between such parameters as the particular shape of the capsule containing the halogen gas, the wall thickness, and the configuration of the waveguide employed to feed the RF energy thereto may be experimentally determined in order for proper impedance matching and attendant maximum power transfer. For example, known techniques such as placing stubs into the waveguide, at various locations and heights, until the power reflected back into the waveguide from the capsule is in a minimum may be employed to determine the optimum admittance/impedance matching between power source and load. In practice, any one of a great variety of pulsing circuits may be employed in the energy source 10, as known to workers in this art.

Turning now to FIG. 3 of the drawings, two oscilloscope traces are shown, with the upper trace designating two excitation pulses derived from the energy fed through the waveguide 24. The lower trace or curve represents the light output from the plasma 23. As indicated at the traces the maximum power of the incoming pulses was I30 watts and the maximum light output was arbitrarily taken to be unity. The reader will observe the extremely rapid decay of luminous output with cessation of exciting power. Each square of the reticulate background of FIG. 3 represents 1 square centimeter. Accordingly, with the indicated scale, it is seen that the time lag between the cessation of exciting radio frequency energy to the halogen gas, and the cessation of illumination from the plasma 23, is much less than 1 millionth of I second.

The apparatus from which the data of FIG. 3 was obtained was a capsule 20 filled with chlorine to a pressure of 9 torr. The beta ray source 27 was 0.1 microcuries of cobalt 60. Also, 0.3 microcuries of heavy hydrogen (H were found suitable for the source. The radiofrequency input power was 9.8 GHz and an X-band waveguide 24 was employed. It was determined that only 40 milliwatts of average power escaped beyond the encapsulated discharge, bearing in mind that the maximum power input was watts. Critical examination of final data showed a lag of 420 nanoseconds at the 2 percent point relative to the cessation of each excitation pulse. It will further be observed from FIG. 3 that not only is there a very small time lag, but that the light attenuation rate in lumens per second closely follows the falloff rate of the excitation pulse. Microwave probing of a chlorine plasma shows that the excita tion rate increases slowly with large increases in exciting power. Theoretical considerations, not necessary for complete understanding and practice of the invention, indicate that the recovery period is proportional to the logarithm of the electron density. Increases of 10:1 in plasma density result in a lengthening of the recovery period by only 10 percent. Such considerations further reveal that the filling pressure within the capsule may vary over rather wide ranges, for example from 1 to 1,000 torr.

What is claimed is:

l. A pulsed gaseous plasma illuminator including,

a. a closed capsule having at least one wall portion transparent to visible light and having at least one wall portion transparent to radiofrequency energy,

. an attaching gas within said capsule,

. means including a waveguide for transmitting to the said gas an RF oscillating electric field,

d. said waveguide having a coupling aperture contiguous to said capsule,

e. said aperture being in the form of a slot through a wall separating a closed end of said waveguide from said capsule,

the outside periphery of said slot abutting said wall of said capsule and said closed end of said waveguide being tapered towards said slot and merging with the inside preriphery of said slot. I I 2. he plasma illuminator of claim 1 wherein said attaching gas is a halogen gas.

3. The plasma illuminator of claim 2 including a free electron source within the capsule. 

2. The plasma illuminator of claim 1 wherein said attaching gas is a halogen gas.
 3. The plasma illuminator of claim 2 including a free electron source within the capsule. 